1. Field of the Invention
The present invention relates to a geared motor unit having a function for limiting a thrust load applied to a motor when transmitting power to the motor via a gear.
2. Description of the Related Art
In an electric motor having a structure for applying a constant preload to a bearing for supporting a rotor, when the preload is not applied to the bearing, the rotor may fluctuate and as a result the motor may be damaged or broken. In a geared motor in which a gear is attached to an electric motor, a thrust load may be generated in a direction so as to cancel the preload, due to the rotation of the motor, depending on the type of gear. When the generated thrust load is larger than an allowable torque, the stiffness of the motor may be lowered and the motor may fluctuate due to disturbance, whereby the motor may be damaged.
In the prior art, a technique for protecting a geared motor from a thrust load larger than an allowable torque has been proposed. For example, JP 2003-322223 A discloses a gear unit wherein an annular projection formed on an inner periphery of a shaft hole of a helical gear is engaged with a groove formed on an outer periphery of a drive shaft to which the helical gear is attached, and the annular projection is disengaged from the groove when an axial thrust force exceeds a certain limit value.
Further, JP S60-129453 A discloses a gear transmission for transmitting torque by engaging helical gears, wherein the gear transmission includes a load cell configured to detect an excess thrust force due to an excess torque and transmit an operation stop signal.
The geared unit of JP 2003-322223 A can be prevented from being damaged by disengaging the helical gear from the drive shaft. Therefore, it is necessary to fit the helical gear with the drive shaft once again, which is troublesome. On the other hand, in the gear transmission of JP S60-129453 A, the operation of a machine including the gear transmission is stopped each time a thrust force larger than a limit value is generated, which may reduce operation efficiency of the machine.
Alternatively, a torque limiter may be used for a geared unit, by which a torque of an electric motor is limited so that a load applied thereto does not exceed an allowable torque. However, in this case, it is necessary to prepare a space for positioning the torque limiter, between the motor and a transmission or between the transmission and a driven object, resulting in an increase in size of the unit.